Pregnancy Test

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Pregnancy test

Definition

Pregnancy is detected by measuring the concentration of human chorionic gonadotropin (hCG) in serum or urine. Human chorionic gonadotropin is a hormone produced by the placenta that supports the corpus luteum after fertilization of the ovum. Production of hCG begins at the time of implantation, and hCG can usually be detected in serum and urine within 10 days after fertilization. The level of hCG in serum and urine is usually above 25 mIU/mL, the cutoff for a positive pregnancy test, before the next expected period. Therefore, pregnancy can be detected reliably within two to three days following the first missed menses using a qualitative hCG test. In addition to diagnosis of pregnancy, the test is used in emergency departments to rule out pregnancy in circumstances in which x ray and other procedures are contraindicated by pregnancy. The test is also used to rule out pregnancy in females with acute abdominal pain that suggests the possibility of ectopic pregnancy (i.e., pregnancy outside the uterus).

Quantitative measurements of hCG are used as an aid to the diagnosis of ectopic pregnancy and trophoblastic tumors. Serial measurements may be used to monitor treatment and recurrence of tumors that secrete hCG. Measurement of hCG is also part of the triple marker screening procedure performed on maternal serum between weeks 15 to 20 to assess the fetal risk of Down syndrome .

Description

Chorionic gonadotropin is a hormone consisting of two polypeptide chains or subunits designated alpha and beta. The alpha chain is identical to the alpha chain of luteinizing hormone (LH), follicle stimulating hormone (FSH), and thyroid stimulating hormone (TSH). The beta chain is identical to that of LH except for the C-terminal end, which contains an additional 24 amino acids. Antibodies made against the alpha subunit will cross-react with LH, FSH, and TSH, but antibodies can be made to the beta subunit that are hCG-specific. All tests for pregnancy utilize antibodies to both subunits, which makes the pregnancy test highly specific for hCG. Chorionic gonadotropin is produced at an exponential rate through week 12 of gestation, often reaching in excess of 100,000 mIU/mL. In a normal pregnancy, the production of hCG doubles approximately every two days during this period. The level falls off sharply after the first trimester to approximately 20,000 mIU/mL, and is maintained at this level throughout a normal pregnancy. Following a normal delivery, the hCG can be detected in serum and urine for three to four weeks. This period may be longer following an aborted pregnancy, especially if a trophoblastic tumor was present.

All pregnancy tests are double antibody sandwich immunoassays. The most commonly used platform, called immunochromatography, consists of a disposable device containing a membrane on which an antibody to one hCG subunit is immobilized. The membrane also contains an antibody to the other hCG subunit that is mobile. The mobile antibody is conjugated to an enzyme, dyed latex particle, or colloidal gold particle. Sample is added to the device and is drawn by capillary action onto the membrane, where it mixes with the mobile antibody. If hCG molecules are present, they bind to the mobile antibodies, forming antibody-antigen complexes. These migrate along the membrane to the region containing the immobilized antibody. The immobilized antibody binds to the other hCG subunit, forming an antibody-hCG-antibody sandwich that remains fixed to the membrane in the reaction zone region. At this point, the dye or gold particles are focused in the reaction zone and produce color, usually in the form of a plus sign or other visible indicator of a positive test. If an enzyme-conjugated antibody is used, a substrate solution is added, which is hydrolyzed by the enzyme to produce a colored product at the reaction zone.

Precautions

In order to achieve accurate results for home pregnancy tests, the manufacturer's instructions must be followed precisely. A significantly higher error rate has been observed with home pregnancy tests than with laboratory tests for pregnancy, which typically have an error rate below 1%. Diluted urine may cause a false-negative result. False-positive results may be caused by heterophile antibodies, medications containing mouse monoclonal antibodies, autoantibodies, and hyperlipemia. In very early pregnancy, the color reaction may be difficult to interpret. In such cases, the test should be repeated after waiting at least 48 hours. If serum is to be used, standard precautions for the prevention of transmission of bloodborne pathogens should be followed.

Preparation

No preparation is generally required for a pregnancy test. However, if urine is used, the first morning urine is the specimen of choice because the urine will be more concentrated after an overnight fast.


KEY TERMS


Ectopic pregnancy —A pregnancy that develops outside the mother's uterus. Ectopic pregnancies often cause severe pain in the lower abdomen and are potentially life-threatening because of the massive blood loss that may occur as the developing embryo/fetus ruptures and damages the tissues in which it has implanted.

Embyro —In humans, the developing individual from the time of implantation to about the end of the second month after conception. From the third month to the point of delivery, the individual is called a fetus.

Hormone —A chemical produced by a specific organ or tissue of the body that is released into the bloodstream in order to exert an effect in another part of the body.

Human chorionic gonadotropin (hCG) —A hormone produced by the placenta of a developing pregnancy.

Hydatidiform mole —A rare, generally benign grape-like mass that grows in the uterus from the remains of an abnormally developed embryo and surrounding tissue. In extremely rare cases, the mole develops into a choriocarcinoma, a malignant tumor that can invade the wall of the uterus.

Implantation —The attachment of the fertilized egg or embryo to the wall of the uterus.

Menstrual cycle —A hormonally regulated series of monthly events that occur during the reproductive years of the human female to ensure that the proper internal environment exists for fertilization, implantation, and development of a baby. Each month, a mature egg is released from the follicle of an ovary. If an egg is released, fertilized, and implanted, the expanded lining of the uterus is maintained. If fertilization and/or implantation does not occur, the egg and all of the excess uterine lining are shed from the body during menstruation.

Miscarriage —Loss of the embryo or fetus and other products of pregnancy before the middle of the second trimester. Often, early in a pregnancy, if the condition of the baby and/or the mother's uterus are not compatible with sustaining life, the pregnancy stops, and the contents of the uterus are expelled. For this reason, miscarriage is also referred to as spontaneous abortion.

Placenta —The organ that unites the fetus to the mother's uterus. The placenta produces hCG, among other hormones, to help maintain the pregnancy. It transfers nutrients and antibodies to the fetus, and waste products from the fetus. After delivery, the placenta, known at this point as the afterbirth, is expelled.


Aftercare

No special care is required after a urine test for hCG. If blood is drawn, discomfort or bruising may occur at the puncture site or the person may feel dizzy or faint. Pressure to the puncture site until the bleeding stops reduces bruising; warm packs relieve discomfort. Women who feel faint should be observed until the condition goes away.

Complications

Tests for hCG levels pose no direct risk to a woman's health. The main risk with a home pregnancy test is a false-negative result, which may be lessened by following the manufacturer's instructions carefully and waiting at least several days after the expected menstrual period before performing the test. A false-negative result can cause a delay in seeking prenatal care , which can pose a risk to both the woman and her fetus.

Results

HCG levels below 25 mIU/mL will give a negative pregnancy test result for all pregnancy test methods. Home test kits use a cutoff of 50 mIU/mL and will be negative below this level. Following miscarriage or abortion, the pregnancy test may remain positive for four weeks or longer. An hCG test performed during this time may be positive in the absence of pregnancy.

The upper limit of normal for a quantative hCG test is approximately 5 mIU/mL. In rare circumstances, such as pelvic inflammatory disease, the hCG level in nonpregnancy may be greater than 5 mIU/mL. Persons with trophoblastic tumors, molar pregnancies, and choriocarcinoma will have greatly elevated levels of hCG. HCG may be found in persons with testicular cancer and other malignancies that secrete hCG or alpha and/or beta subunits. Quantitative hCG measurement is useful in detecting hCG-secreting tumors. Periodic measurements are useful in evaluating treatment and monitoring patients for recurrence. Maternal serum hCG levels are increased by approximately 25% above normal for the gestational age in Down syndrome pregnancies and in some other trisomy syndromes. In ectopic pregnancy, hCG levels are lower than normal, and the hCG doubling time is less than expected. Minimum hCG increases between timed hCG measurements in the first trimester are:

  • two measurements one day apart: 29% increase
  • two measurements two days apart: 66% increase
  • two measurements three days apart: 114% increase
  • two measurements four days apart: 175% increase
  • two measurements five days apart: 255% increase

Recovery of a lower than expected increase is evi dence of ectopic pregnancy. Decreases in hCG are seen in spontaneous abortion.

Health care team roles

If serum is used, a phlebotomist or nurse collects the blood specimen. A laboratory scientist, nurse, physician assistant, or physician can perform the pregnancy test. The result should be reported to the physician who orders the test. Quantitative hCG tests are ordered and interpret ed by a physician and performed by a clinical laboratory scientist, CLS(NCA)/medical technologist, MT(ASCP) or clinical laboratory technician CLT(NCA)/medical lab oratory technician MLT(ASCP).

Resources

BOOKS

Chernecky, Cynthia C, and Barbara J. Berger. Laboratory Tests and Diagnostic Procedures. 3rd ed. Philadelphia, PA: W. B. Saunders Company, 2001.

Kee, Joyce LeFever. Handbook of Laboratory and Diagnostic Tests. 4th ed. Upper Saddle River, NJ: Prentice Hall, 2001.

PERIODICALS

Bastian, L.A., et al. "Is This Patient Pregnant?" Journal of the American Medical Association 278 no. 7: 586-591.

Peredy, T.R., and R.D. Powers. "Bedside Diagnostic Testing of Body Fluids." American Journal of Emergency Medicine 15 no. 4: 404-405.

Victoria DeMoranville